Car: 2005 Outback L Automatic Pertinent Modifications: Description of the problem: P0131... please don't jump to conclusions here. Recent maintenence or modifications: PO replaced 2.5 (ej253) with 2.0 (ej203) Alright, so I picked up this outback knowing it was throwing an O2 CEL. I ran to the auto store and grabbed a scanner and found P0131 (front O2 volt low). On my way out, I snagged a replacement front O2 and jacked the car up to find the current O2 sensor looks pretty new (hardly any grime on it). So I returned the O2 I got because I wanted to investigate this before fighting the old one out (side note, WTF Subaru, why did you make the front O2 so hard to get out of the N/A cars?). Anyways, I borrowed a Tactrix cable thinking it might be related to the displacement and pulled a few logs and was pretty surprised to find that the front O2 seems to work fine for the first few minutes of the drive. I reset the ECU (clearing p0131) and left for a drive. Everything seems fine until I let off the throttle going down a hill and the AFR in the logs ramps from ~15-16 (seemingly normal afr for closed throttle) all of the way to 20.44 within a 200 ms. It sits at 20 aft for about 2500 ms before switching straight to 14.7... Sometime during this time period, the car will trip P0131. What is weird is that if I turn the key off and back on quickly, the O2 sensor starts reading the correct values again (despite P0131 staying thrown). It will work fine for almost a minute before it goes back to 14.7 while driving. To me it seemed like it was a physical issue with the wire except it works immediately after a key cycle. I tugged on the wires but didn't see a change when the ECU reads 14.7. I also thought it could be a failing o2 but the PO mentioned it was recently replaced by the owner before him (accurate based on what I saw under the car) to attempt to fix the same code. I don't know if the ECU shuts power off to the O2 when It thinks its bad, but that seems to be happening in this case. Anywho, I am going to do some research on this but wanted to see if anyone had any issues like this before. I'm trying not to throw money at it and the front O2 is too expensive and much of a PITA to replace on a whim.
Did you check voltage and resistance at the sensor? If not, I'd start there and work my way back to the ECU.
From my experience, at least on turbo models, Subaru's tend to be kinda picky about using OEM electronics so if it's not an OEM sensor, it could be suspect.
My multimeter grew legs but I'll check that when I find it.. That was my first thought but I was suprised to see it work if the key was quickly cycled. I couldn't see the brand on it but maybe. Seemed strange that it was apparently out in to fix the same code but didn't. That lead me to looking at the ecu itself. But your post actually seems like a good lead. I know there are voltage ranges in the ecu that can trip a cel. This is actually the reason I went with a cobb flex instead of gm.. The cobb sensor operates within 0.5-4.5 range and can be configured via tune. I bet lean conditions could let a non-oe sensor read lower than the threshold stock ecu threshold.. Maybe when the threshold is broken, the ecu stops sending the o2 power and sets default to 14.7..
This is a good point: Def seems like your soft reset is pulling it out of closed loop. While you're at it, it might be a good idea to double check the other sensors if you haven't already. All the jdm swaps I've done have required ecu specific cam/crank sensors as well as triggers.
I'll absolutely second this (again, turbo experience only here). The up front cost sucks but the frustration caused by non working parts is probably worse.
PO swapped all cam and crank gears and put the 2.5 intake back on. Only the block/heads is 2.0. I actually got everything from the swap too if I need to go back to the drawing board. My logs didn't capture raw voltage of the sensor.. I'll try to look at the map with the tactrix and see if I can play with the thresholds at all. If not I'll replace the front o2 with oem. It's just such a pita to get out. A free beer for anyone who has a better tool and a free evening.
I will say I remember the wire being yellow when I first looked at the car and can't seem to find a denso sensor with that same color wire, while researching what I can while at work... I also had a chance to dig into the logs again and the WAY the o2 fails is strange. I put the logs in google if anyone wants to see them: https://docs.google.com/spreadsheets/d/18YDgtRR7Z_6WLPJhcIGYtIR2WxRkhTzH4ZqLJEXfG4M/edit?usp=sharing It almost seems like the O2 is pushing a threshold during deceleration but still works after, but I didn't notice it working because the ECU is forcing open loop. With that being said, I think I will try forcing P0131 off to see if the ECU/O2 are just intermittent. Either way, I found a Denso sensor and possibly a better wrench to use, which I will get tonight.
Mike, what did we tell you about offering beer in exchange for these people's tools? (haha) good luck in finding and squashing your gremlin.
I'm mad it was a simple fix. Regardless. For anyone doing an o2... This is hands down the best tool I've bought all year.. And I swapped a motor this year.
